The first step of vision takes place in the retina where light is captured by the outer segment of photoreceptor cells. This light-sensing organelle is unique in that it is a modified primary cilium that is composed of a specialized group of proteins involved in visual signal transduction. These proteins are synthesized in the cell soma and use discrete intracellular trafficking mechanisms to be delivered to the outer segment. These trafficking mechanisms are critical for development and general maintenance of photoreceptors, as the outer segment is a dynamic structure that must be continuously renewed. During the mentored phase I will complete three ongoing, interconnected projects that address how intracellular trafficking mechanisms are established and required for outer segment stability. First, I will decipher how the features of rhodopsin and its trafficking pathway are necessary for outer segment formation. Second, I will determine the scope of Arf4 involvement in rhodopsin trafficking to outer segments by analyzing a rod-specific Arf4 knockout mouse. Finally, I will identify and refine active targeting domains encoded within proteins localized in the outer segment. This comprehensive approach will facilitate my transition to the independent phase, in which I propose to elucidate how signaling proteins are segregated between the disc and the plasma membrane subdomains of the outer segment. My proposal has broad implications for understanding the pathobiological processes underlying cases of inherited retinal degenerations, many of which are caused by mutations affecting protein trafficking to the photoreceptor outer segment.